Story: Whales

Page 8 – Strandings: whales and dolphins

New Zealand is one of the main locations for whale and
dolphin (cetacean) strandings. Records of strandings have
been kept since 1840. Even though early records are sketchy
and were usually of strandings near centres of population,
they do show trends. About 13,000 individual cetacean
strandings have been recorded, and by 2006 over 2,000
creatures had been successfully rescued.

Certain areas are stranding hotspots: Ōpoutama (Māhia
Peninsula), Golden Bay and the Chatham Islands. Some species
are attracted to particular areas: sperm whales to Kaipara
Harbour and pilot whales (which belong to the dolphin family)
to Doubtless Bay, Ninety Mile Beach and Stewart Island. In
1998 Doughboy Bay, western Stewart Island, was the scene of
328 pilot whales stranding.

Just six species account for 88% of strandings: the
long-finned pilot, sperm, false killer, pygmy sperm, Gray’s
beaked whales and the common dolphin. Three of these species,
the long-finned pilot, false killer and the sperm whale, are
mass stranders. In 207 incidents, more than 9,000 long-finned
pilot whales have beached themselves, with just over 2,000
being successfully refloated. In 1918, New Zealand’s (and the
world’s) worst recorded stranding occurred when around 1,000
pilot whales came ashore at Long Beach, Chatham Islands.

Pilot whales: common stranders

Pilot whales are members of the same family as dolphins.
The long-finned whale (Globicephala melaena) is
larger than the short-finned (Globicephala
macrorhynchus), and more common around New Zealand. Both
frequent deep water, which accounts for the difficulties they
experience when they encounter unfamiliar shallow waters.

Why do whales become stranded?

Scientists believe there are three main reasons why New
Zealand’s coastline is such a hazard for cetaceans:

Certain beaches such as Golden Bay are ‘whale traps’,
with protruding coastlines and long, gently sloping
beaches. These are usually the locations of mass
strandings. Offshore cetaceans, such as long-finned pilot
whales and beaked whales unfamiliar with the coast, are
likely to get into difficulties here.

Toothed cetaceans use echolocation to navigate, but
this might not work on gently sloping beaches. Scientists
using navy echo-sounders receive a weak echo from these
beaches compared to steep, shingle boulder beaches or rocky
coasts.

Local oceanic conditions may account for strandings.
Golden Bay and Hawke’s Bay have currents that appear to
explain strandings there.

There are other possible causes:

Cetaceans may become stranded for biological reasons.
Theories abound, from parasitic worms in the brain
affecting co-ordination and balance, to whales being
trapped as they follow prey inshore.

Predators such as orcas or sharks may force the marine
mammals inshore. In a 1993 pilot whale stranding at Golden
Bay, orcas were seen patrolling outside the spit. The
whales might have sought sanctuary in shallow waters
because of the threat.

Deep-water toothed whales with strong social bonds
become stranded in a group more frequently than other
species. If an individual is sick, or old and dying, the
rest of the herd will come to its aid, even if they
endanger themselves by venturing too close inshore.